If you stopped someone on the street here in the US and asked them what the most common building material in the world was, most likely their immediate answer would be wood. But in truth the most common material is earth and that has been the case for some thousands of years. Only in industrialized countries is wood the predominant building material and this is a fairly recent situation, created by the way wood as a material has lended itself to industrial production and the evolution of a professional building trade hegemony.

However, in recent years a great variety of earthen construction technologies have become popular among the community of 'green' and 'organic' architects and Soft-Tech advocates who tout it as a more sustainable and natural alternative to conventional construction technology. Through their efforts many forgotten building methods have been rediscovered and many new earthen building techniques introduced. Many virtues are claimed for earth as a building material, including high thermal mass, very low environmental impact, hygroscopicity and permeability supporting a healthier interior environment, simple low-energy building methods readily suited to the DIY builder, universal availability of building material, and -most important of all- it's 'dirt cheap.' But despite a seemingly endless array of beautiful 'green' homes built with this material, there has been no mainstream acceptance in the industrialized world and this is something that has puzzled me for a long time -until I started learning some of the nuts and bolts details of the various earthen building technologies.

As a non-toxic building material earth has a lot going for it. Assuming the source of earth is uncontaminated by industrial pollution, free of heavy amounts of natural contaminants like mold and toxic minerals, and free of naturally occurring radioactive materials, it is one of the the most practical natural non-toxic materials. Its combination of high thermal mass, hygroscopicity (the ability to absorb and release moisture), and permeability/breathability offer the potential for a very healthy indoor environment. It is one of most widely tolerated of all non-toxic building materials and is the basis of some of the most healthy homes in the world. And yet it is, in fact, rather rarely used for non-toxic homes. Why? For the same reason it has failed to become the mainstream building technology Soft-Tech advocates have hoped for.

There are 12 basic techniques of earthen construction and all of them suffer from the same critical problem; very high manual labor overhead. And under the labor cost driven economics of industrialized nations this makes for a very high cost of construction. Thus this class of architecture has often been referred to as the housing of choice for the idle rich and idle poor. It's generally only practical for those with a lot of money or a lot of time on their hands. When true labor cost is factored in, earthen based homes can cost many times as much as a conventional stick-built home. But if this is the case, why is this housing so often referred to as 'dirt cheap'? Because the advocates of this form of construction routinely overlook labor cost and use the material cost alone as the basis of its supposed economy. Outside of the US Southwest, there basically are no contracting services for earthen construction so if it's used at all it's almost always done as DIY projects or as the result of classes held by Soft-Tech advocates to teach these building methods. That labor is treated as free. But these are very special situations, far removed from the realities of mainstream housing. Most people have neither a pool of volunteers to help them build a home or great amounts of free time. And with the styles of architecture common to these building methods limited to use in remote locations because of their radical appearance and conflicts with local building codes, any projects based on them would tend to be conducted far away from their builders likely previous home locations. Theft and vandalism are as big a problem in rural areas of the US as in the inner cities so leaving uncompleted buildings unattended for long periods is impractical.

What is it that makes earthen construction so labor intensive? Let's examine some of the more common methods and consider their pros and cons.

Adobe: This is the most traditional form of earthen construction in the US with a history of some several thousand years of use among the native Americans as well as well as several hundred years of use by Spanish settlers. Worldwide, this technology may be in excess of 10,000 years old and was ubiquitous among all early civilizations of Asia and Africa. The specific characteristic of adobe brick is that it is a wet cast air dried brick molded by hand or in simple forms and bonded with an adobe plaster. Adobe bricks come in several types; unstabilized and stabilized, air and kiln dried, hand cast or machine extruded. Stabilized adobe uses an admixture to the earth in order to provide greater strength and uniformity. Cement is a common admixture but so is asphalt, which is highly toxic. Traditional stabilizers are plant derived, such as prickley-pear cactus juice. Organic matter in the form of straw and plant or animal fibers is sometimes added as a stabilizer, both to bricks and the adobe plaster.

Modern adobe home construction typically begins with a concrete curtain or slab foundation on which the walls are built and which are intended to provide both a stable support and a barrier to moisture seepage from the ground. Compacted stabilized earth and rubble foundations are also used. Walls are built up in brick courses to the desired height and topped by a cast-in-place concrete bond-beam on which is set roof beams or trusses supporting one of several different roof systems. With pueblo style designs peeled log vigas are used to support a flat compound roof composed of thin planks or sticks called latilla topped by insulation panels and a roof emembrane akin to that of industrial buildings. In the native tradition, vigas were employed as a perpetually reusable structural component, large lumber pieces being somewhat rare in the deserts. With Spanish style designs a fired clay tile pitched roof supported by a truss system of heavy wood beams may be used. In contemporary designs rectangular wood or truss joists may support a flat compound roof with metal plate or dovetail board or a wooden or light gauge steel truss system may support pitched metal or shingle roofing. Adobe walls can be left unfinished or finished with an adobe plaster. Such finishing is necessary with the use of unstabilized adobe and must often be frequently renewed depending on wear due to yearly weather. Interiors may be finished with adobe plaster, adobe plaster with a rubbed beeswax surface, stucco or smooth plasters, wooden paneling, or surface mount framing supporting drywall.

Adobe by itself has high thermal mass but poor insulation value. As a result it has become common for modern adobe construction to include insulation. This has taken the form of externally applied foam panels or, more recently, double-wall construction where a gap in the wall is filled with sprayed foam insulation. But while this improves the thermal performance of adobe construction, it comes at the sacrifice in permeability since these insulation materials are impermeable. Thus the health value of the construction is reduced.

Since in some parts of the country adobe block is mass produced in a factory setting it can reduce some of the labor overhead at an increase in material cost. The potential for mass production of adobe blocks in advance of construction and without regard to specific building use is one of the key advances of the technology over simpler earthen construction methods and, in fact, adobe block manufacture was one of human civilization's first industries. However, the very large number of blocks used and their significant individual mass still makes for a very labor intensive process. Adobe is the only earthen construction technique for which conventional contractor services exist in the US, and this only in the Southwest. Their cost for adobe construction is roughly $125 per square foot, making a modest 1000sf bungalow a $125,000 investment before finishing, land, and utilities costs.

Mechanically Compressed Earth Block: Second in popularity to adobe is mechanically compressed earth block. Basically similar to adobe, it differs primarily in the method of block fabrication. Instead of a wet mud being loaded into molds and air dried, this type of block uses a drier mix of earth with a stabilizer such as cement which is loaded by bin into a molding machine, the hand operated press machine commonly referred to as a 'Cinva ram'.

Cinva ram

The compression process reduces the volume of earth and rearranges clay particles to form a more stable material matrix. Mechanically compressed earth block offers a higher geometric uniformity than adobe and can be produced at a much faster rate. The Cinva ram was invented primarily with the use of this material in Third World countries in mind, restoring traditional earth construction methods that had been supplanted through the machinations of exploitative western industrial interests. But, simple as this technology is, it has become quite sophisticated over time. One of the most advanced of these hand operated rams, the Auram 3000 by the Aureka company, is capable of producing a great variety of block forms with a single machine. In particular, it can produce an assortment of hollow blocks which improve upon adobe by increasing insulation value, allowing insertion of reinforcement rods and utilities runs. It can also produce blocks forum columns, hybrid concrete beams and floor/roof systems, and can produce blocks with interlocking grooves allowing for construction without mortar, a thinset cement 'glue' used instead. More than just a block maker, it is a comprehensive construction system in one tool. Extensive research and development of this technology has been made throughout the world and particularly by the Earth Unit development group of the Auroville eco-community in Southern India. The relatively low cost of these earth ram machines makes them attainable to most DIY builders, though -despite being readily available by mail order- they are curiously less common in the US than elsewhere in the world.

Compressed block construction is essentially the same as adobe construction but with the greater diversity of possible block forms it is much more versatile in terms of possible design and decoration. A diversity of roof systems including barrel vaults, domes, ferro-cement, corrugated metal, hybrid earth block and concrete, as well as all those common to adobe are possible with this material. Unfortunately, not much is different in terms of labor overhead. The chief labor savings offered with this technology is the elimination of mortar through interlocking blocks. This can reduce the labor by 1/3rd to 1/2. All in all, it is generally superior to adobe.

Cob: This technology is the predecessor to adobe and has its equivalents in all the regions where adobe is common. However, it also has a tradition in the seemingly unlikely location of the British Isles, from which derives its contemporary name. Cob construction is a more-or-less continuous material processing and wall building operation, labor divided into the mixing and forming of cobs -loafs of moist mud/straw mixture- and the stacking of courses along wall lines. Construction begins with a foundation system akin to adobe but often much higher off the ground -particularly in the British tradition where damp climate makes ground seepage a critical issue. Courses of damp cobs are stacked along wall lines with one to a couple layers done per day. Modern cob construction may top the walls with a concrete bond beam, use a wooden bond beam, or a separate roof frame supported on a post and beam system. In the British tradition, walls are topped with a wooden bond beam and a heavy beam truss roof system sheathed in thatch and with a good overhang to protect walls from run-off. More contemporary cob homes typically use lighter wood shingle roof systems.

The laying of cob courses is a very sculptural process and contemporary cob builders employ a very organic design with round or flowing walls and many interior fixtures integrated into the wall forms. The use of rough gnarled shape lumber in fixtures and structural pieces enhances this organic aesthetic. This new form of organic cob design has made it very popular among Soft-Tech advocates, the homes having a very fanciful look compared to cob homes of the British tradition or, for that matter, the rounded edged rectilinear forms typical of Southwestern adobe. But this freedom of form is a double-edged sword. it's as easy to make something ugly as it is to make something attractive and effective results can require a fair degree of artistic talent. It's also easy to make mistakes leading to structural failure unless one remains conservative in the scale of the structures.

Cob shares adobe's poor insulation value and the application of external or internal foam insulation is generally not an option. It also suffers from very extended drying-out times before interiors can be finished. In the colder damper climates dry-out periods can be in excess of a year and exposure to early freezing can have devastating results. It does not lend itself well to incremental construction over very extended periods as exposure to the elements without roof or plaster protection can quickly destroy unfinished structures. It also has a fairly high maintenance overhead. Similarly, it is especially susceptible to damage from plumbing failures.

Rammed Earth: This technology has long been favored as an approach for multi-storey earthen construction. China and France seem to have the best known building traditions for this technique. Rammed earth construction is conducted by erecting wooden or metal forms for the walls and filling them with a cement stabilized earth mix which is compacted by pounding with hand tools or with a mechanical compactor. Metal rebar is often added to further increase strength. Different kinds of earth or mineral compounds are sometimes added to each earth layer for the sake of decoration, the striated layers of color in the finished wall reminiscent of natural sandstone and considered one of this technique's most attractive features. The finished walls are massive and monolithic, offering high strength, high thermal mass and higher insulation value than block/brick construction. High load bearing strength allows for multi-storey structures, usually based on floor decks supported by massive wood beams. The same types of roof systems employed with adobe can be used but metal or shingle roofing with a wide overhang is more typical. No surface finishing is required except for aesthetic effect. A beeswax finish is typical for the interiors as it allows the attractive wall texture to remain visible.

Rammed earth is one of the most labor intensive earthen construction processes of all because of the combination of heavy form erection, wall pounding, and the mixing and moving of large volumes of earth. But it's also one of the most beautiful looking due to it's sandstone-like appearance and large wall thicknesses.

Rammed Earth and Tires: This technology has been made world famous through its use as the basis of the Earthship sustainable housing system. Intended as a means to reduce the scale, if not the volume, of labor involved in rammed earth construction while at the same time recycling a ubiquitous waste product. Rammed earth and tire construction is based on the preparation of old tires as rammed earth bricks by filling them with earth and hammering it with a sledgehammer to compress it. These tire bricks are then stacked in alternating courses with rebar nailed into the top course to bond a wood or concrete bond beam. 'U' shaped and cylindrical forms are typical to provide further strength and intuitive engineering. A roof system similar to that of pueblo style adobe structures is used but in the Earthship designs this is elaborated to support a roof membrane for the collection of rainwater in integral cistern tanks and for the integration of a passive solar window wall running the full length of the basic structure. Finishing is by a thick adobe plaster with the lumpy tire wall surface smoothed by the addition of aluminum cans pressed into their gaps. The technique works well for free standing and earth bermed structures but tends to be limited to single storey or terraced structures.

The chief advantage of this technique is that it eliminates the formwork and precarious form-top pounding work associated with rammed earth construction. This does reduce equipment cost compared to rammed earth. But it otherwise does nothing to save labor overhead, just breaks it up into smaller pieces. There is still a great deal of work in pounding tires full of earth with a sledgehammer then lifting the resulting hundred pound object into place. Thus this technique lends itself to the lone individual builder -but still one with a whole lot of time on his hands! The impermeability of the tires affords the capability for earth berming and allows for elegant integration of homes into the landscape. But this also eliminates the permeability and hygroscopicity that are key virtues of earthen construction. As a non-toxic construction method it is ill-advised. I was once told by the Earthship's original designers that encapsulation by adobe plaster stopped outgassing from the tire material itself because it plocked off UV exposure. But this does nothing for the other contaminants the tires are impregnated with by running on asphalt and being soaked in fuel, oil, and exhaust. So it's probably not safe to use for a non-toxic home. However, other recycled items do have potential for use in the same way. For example, polyethylene 55 gallon drums that have not been used to store toxics may be suitably non-toxic in this form of use and can be packed with earth and stacked in much the same way. They would, however, be too heavy for people to lift and position without mechanical assistance.

It's useful to note that the virtues of the Earthship design are not exclusive to this rammed earth and tire construction. Almost all other earthen construction techniques could be applied to this same type of design -at least as far as free-standing structures are concerned.

SuperAdobe: This recently introduced technique is the brainchild of architect Nadar Khalili and the Cal-Earth Institute , though it has its origins in both Middle Eastern earth building techniques and the earth bag building methods developed by other Soft-Tech enthusiasts in the 1970s. The key to this technique is the use of continuous sandbag tubing commonly used in the fabrication of sand bags for flood control, military trenches, retaining walls, and similar tasks. Structures are constructed by laying out coils of the tubing which is filled with a cement stabilized earth or sand mix as the coil is layed down. Each coil is locked to those below it by coils of barbed wire. Coils with a greater amount of cement are used as a foundation and structures are typically built partly below grade so that the volume of soil excavated can provide sufficient material for the construction. Structures are built-up in the manner of coiled pottery with domed and barrel vault shapes predominate. Smaller scale details are formed by using smaller lengths of tubing down to the size of small block-like bags which are stacked in a brick fashion. The completed structures are covered in a thick coating of adobe plaster inside and out supported by metal or plastic mesh. This is critical not only for appearance but because the material of the bags can deteriorate rapidly when exposed to UV light.

SuperAdobe has rapidly gained attention of the Soft-Tech community and has been used to build some large community buildings in the desert areas of California. It has also been the focus of much interest at NASA which has sponsored research into its use with regolith as the basis of shelter systems for future Lunar and Martian settlements. SuperAdobe offers great reduction in construction time and labor overhead compared to other earthen construction techniques. This is because the sandbag tubing is simply filled with loose earth material as the coils are laid in place, eliminating steps like brick fabrication and mortaring and the lifting of any heavy components. And the skill level required is very low. This affords it, perhaps, the lowest labor overhead of all known earthen construction methods. And soil composition is not as critical an issue as it is with most other earthen building techniques. It is suitable for earth bermed construction in a shallow slope situation and is ideally suited as an easier to use alternative to rammed earth and tire for Earthship style architecture. However, it is not without some significant limitations. Curved forms are critical to structural integrity with this technique. Long straight walls are unstable with this kind of structure and so designs tend to be limited to domes, cylinders, and barrel vaults. Multi-storey structures may only be possible with separate internal mezzanine structures, except in the possible case of very small cylinders and domes or walls of great thickness relative to the supported deck area. Windows and doors are typically limited to arched or round shapes because of the inability to integrate lintels into the structure. The structure has to coil over such openings. Aging of the sandbag material is a critical issue because it is normally designed to deteriorate or be biodegradeable and failure of this material can lead to critical structural failure. While the structures can provide their own monolithic roof forms, adobe does not weathering well and the structures may require additional protection in the form of elastomeric coatings in all but the driest climate conditions. And for all its potential labor savings, it is still far more labor intensive than conventional stick frame construction and a small home will typically require a crew of some dozen people to complete within a construction period of several weeks, or for a lone individual a period of some months. Still, the intuitive nature of building and design by this technique is extremely compelling.

As a non-toxic building material SuperAdobe should be adequate depending on the exact nature of the sandbag material employed. Usually a form of polyethylene, this material has almost no outgassing potential when blocked from UV exposure. But to my knowledge no EI has ever lived in one of these structures for any length of time. The technique is too new and volunteer labor only readily available to the charismatic leaders of Soft-Tech organizations.

Straw Bale: This is ostensibly an earthen construction method only because of its use of adobe plaster. One of the most popular of the 'green' building technologies among advocates, it is actually a comparatively recent invention unique to the US, being the product of 19th century plains farmers ingenuity. The building process is akin to adobe construction, starting with the same type of foundation and using large bales of straw as the 'bricks'. The bales are stacked like large bricks and 'nailed' in place using long metal pins -often threaded rod- tension cables or special gripping plates then are covered in mesh lathe attched by wire pins and an adobe and straw plaster. Straw bale is used either as a load bearing wall system -usually limited to a single storey- or as an in-fill wall for heavy wooden, steel, or concrete post and beam systems. As a load bearing wall system the bales must be topped with a bond beam of heavy lumber, steel, or concrete. A recent innovation uses bales with holes cut in them as forms for a reinforcing concrete frame, much like a giant version of insulated concrete forms. Wide overhang roofs are typical with straw bale, commonly using metal or shingle, but sometimes a pueblo-style roofing system may be used. In certain regions building codes restrict its use to in-fill construction and in general this seems to be the preferred approach, though that tends to eliminate any economy it might offer over other materials and methods and hence limit its use to the more well-off -but then, this has been the perennial problem for all green architecture.

The low skill requirement coupled to a high insulation value and fast construction are key attractions for this technology. It has enjoyed a very wide range of climate conditions for its use, proving effective in everything from desert to humid to cold winter regions. It is a less time consuming form of construction but this is only because of the large size of the bales, which are not of insignificant weight and limit the geometry of structure greatly. Labor overhead is generally less than adobe but not if extra effort must be spent on a post and beam frame. Its value as a non-toxic building technology is subject to debate. There is a possibility for poorly built structures to be subject to pest and mold infestation and as a permeable wall system it is critical that the straw used be from exclusively organic farms and thus pesticide-free. This tends to greatly complicate its use. And the cost of straw bales is wildly variable in general, since they are not readily available in most places. Despite these drawbacks, green architects and Soft-Tech advocates in the US are very enamored of it, perhaps because it is the only exclusively American invented technology.

Waddle & Daub: Known mostly from its traditions in European vernacular architecture, this technology actually has many equivalents throughout the globe. Essentially, it is the hand application of and adobe-like mud plaster to a lathe structure formed of any of a great variety of materials, mostly of plant origin. It is the predecessor to the lathe and plaster interior wall systems which became the standard in western construction in the 17th to 19th centuries until the introduction of drywall products. It is used primarily in the form of an in-fill system for post and beam structures and has neither insulation value or thermal mass but does enjoy good permeability. It is occasionally used as a complete building system using lathe frames similar in character to that of free-form organic ferro-cement construction and, in fact, a ferro-clay construction system like this has ocassionally been used in modern times. But the lack of effective structural strength using adobe has tended to limit that kind of use. Waddle & daub is fairly simple to perform but since it is of so little use as a structural system alone its labor overhead cannot be considered outside of the labor overhead for the whole frame system it is used in.

Looking at all these systems it's clear that there really is no low labor option with any form of earthen construction. Probably the most labor efficient of these methods is compressed earth block because of the ability of the newer more sophisticated mechanical ram and interlocking block systems to provide a complete self-insulated structural system including roofing using this material alone or with judicious use of concrete. The extensive work done with this material -especially in India- has also produced some effective labor savings techniques, such as the clever ferro-cement roof panel system demonstrated by the AUM-house disaster relief housing shown below.

AUM-house with ferro-cement panel roof - Auroville India

There is, however, one very compelling alternative to these earthen systems that produces essentially identical architecture but which has a greatly reduced labor overhead and very fast construction time. This is construction based on a material known as pumice-crete.

Pumice is a naturally occurring light porous igneous rock that is found in great abundance in many parts of the world. Thought defnitely more expensive than earth, it is generally low in cost with a typical cost in the area of $25 per ton or about $15 per cubic yard. Mixed in a ratio of 10:1 with cement it produces a pourable mix that is roughly equivalent in strength to compressed earth. Pumice-crete walls have moderate thermal mass and a high R-value in the 40s. Building method is essentially the same as for rammed earth except that it can readily employ the flat roofing systems of pueblo style construction or can itself be used as a roofing system in the form of tiles or monolithic slabs supported on integrated beams. The cast pumice matrix is readily 'carved' with hand tools and can be sculpted into all the organic forms and intergated furnishing features typical of other earthen techniques, though the shape of primary structures are limited by formwork. Plastering is necessary on all surfaces but the material will not deteriorate due to moisture as with earth, thus making it suitable for use in all climates and regions. It is readily intercompatible with other earthen based and concrete materials.

This could be called a 'nearly-earthen' construction method and it is definitely both eco-friendly and completely non-toxic. Labor is low and construction time very fast with this material because of its low weight and the fact that a whole home can be produced in a continuous process. But the scale of labor is relative to the scale of a single material pour done in a single day. In other words, you can't stop a pouring process in the middle and start it up again later. It has to be completed in one process in one day. The bigger the structure, the more people it takes because the amount of time must always be roughly the same. So the labor is relative to the unit structure size. Modular design ameliorates this limitation by simply reducing the unit scale of structure that must be produced at once. But for a person working alone this may equate to a scale of structure too small to even function as an effective individual room. So while net labor may be low, the nature of the construction limits the practical workforce to a minimum of a good handful of people.

From my personal standpoint I am attracted to compressed earth block because it is potentially something I could 'do' by myself, even though my poor health might mean taking ten times as long as the average healthy person. I could potentially afford the simple hardware and while it's net labor overhead is high it lends itself to slow incremental work with a very small unit component scale. Unfortunately, that doesn't really help me much since I can't perform any of this work when I'm 1000 miles away from where I need to go and have no practical option for temporary housing.

Pumice-crete seems a much more practical option even though, technically, it would be more expensive. It may require a fair handful of volunteers to perform the work but it's far easier to get a handful of people together for a week than for a month. Unfortunately, since its costs must be paid for up-front it will still be too expensive for me alone. The basic structure of a 1000-1500sf home is likely to cost about $5000 just for the pumice-crete material and construction and not including the foundation, roofing, finishing, land, and utilities which would effectively quadruple that.